Anthrax is primarily a disease of sheep, cattle, horses, goats and swine caused by Bacillus anthracis, a gram-positive spore-forming aerobic rod that produces exotoxins. The organism is transmitted to humans by inoculation of broken skin or mucous membranes causing cutaneous or gastrointestinal infections, or by inhalation, causing pulmonary infection. Anthrax is a rare occupational disease of farmers, veterinarians and wool workers. B. anthracis is designated as category A biothreat agent because of the ease of weaponization of spores and its persistence in the environment. The bioterrorism threat from inhaled B. anthracis spores has increased the need for effective treatments and preventions of this deadly disease.
Inhalation anthrax is the most deadly form of the disease. The incubation period ranges from 1-43 days, with a mean incubation period of 16 days (Cieslak et al., 1999). The standard course of antibiotic treatment post-exposure is 60 days, but the length of treatment for full protection is currently unknown. The mean duration of non-specific prodromal symptoms is 4.1 days, and the mean duration of the fulminant phase is 1.1 days, after which the death rate approaches 100% in the absence of treatment (Holty et al. 2006). The events leading to death from anthrax in humans are not well understood (Jernigan et al. 2001), but the terminal stages of the disease involve bacteremia with vascular damage and injury to multiple organs. Anthrax pathogenesis has been reviewed (Dixon et al. 1999).
B. anthracis produces two exotoxins, Edema Toxin (EdTx) and Lethal Toxin (LeTx). During the initial phases of the disease, the toxins cause destruction of the lymphatic tissue, helping the bacteria gain access to the blood stream (Dixon et al., 1999). They also impair the function of leukocytes that are crucial for phagocytosis and mounting an immune response (O'Brien et al. 1985; Comer et al. 2005; During et al. 2005; Erwin et al. 2001; Fang et al. 2006). EdTx is composed of Edema Factor (EF) and Protective Antigen (PA), and LeTx is composed of Lethal Factor (LF) and PA. As single entities, none of these proteins are known to have any lethal effects. PA is the essential component for EF and LF entry into target cells. PA binds to specific receptors on the host cell surface and after activation by a furin-like protease, forms a heptamer complex and binding sites for either EF or LF are created. The complex (PA+LF or PA+EF) is then taken into the cell via clathrin mediated endocytosis. Upon acidification in the endosomes, the PA heptamer changes conformation, inserts in the membrane forming a channel and allows the two factors to enter the cytosol.
Victims of inhalational anthrax do not experience significant symptoms until a late stage in the disease when they are close to sepsis and toxemia. Antibiotic treatment is largely ineffective at the symptomatic stage in preventing death (Holty et al. 2006), in part because antibiotics do not target the anthrax toxins. To be maximally effective, antibiotic therapy must be initiated within hours of exposure to aerosolized B. anthracis spores, prior to the onset of symptoms (Holty et al. 2006). However, in the event of mass exposure to anthrax spores, as could occur in a bioterrorist attack, treatment would most likely not begin until 3 to 6 days post-exposure, on average, owing to the length of time required to identify potential victims and distribute stockpiled medication (Brookmeyer 2005). At that point, upwards of 25% of anthrax cases following exposure to a lethal dose of anthrax spores would fail to be prevented by antibiotics alone (Brookmeyer et al. 2005). Anthrax vaccines, which over the course of several weeks stimulate the immune system to mount a protective response against PA, are effective in pre-exposure prophylaxis, and can afford some protection from breakthrough infection arising from germination of residual spores after withdrawal of antibiotic therapy. Anthrax vaccines are ineffective when used alone in the post-exposure setting.
An area of unmet need for anthrax is fast-acting medical countermeasures to infection with antimicrobial-resistant strains. The development of such agents would be of great benefit.